Programmed gelation of polymers using melamine resins

ABSTRACT

A process for use with a polymer which is crosslinkable by reaction with an amino resin, which comprises the steps of determining a period of time within which full gelation of the polymer is to be achieved; preparing a gel-forming aqueous-based mixture comprising the polymer, a rapid amino resin crosslinking agent and a delayed amino resin crosslinking agent, the gel-forming aqueous-based mixture capable of complete gelation within the period of time so determined; partially gelling the aqueous-based mixture by reacting the polymer with the rapid amino resin crosslinking agent which is effective to complete the partial gelation by crosslinking the polymer within four hours; and fully gelling the aqueous-based mixture by reacting the polymer with the delayed amino resin crosslinking agent which is effective to complete the full gelation by crosslinking the polymer within the period of time so determined. A method of enhancing the recovery of oil from a subterranean oil-bearing formation is also provided.

This is a division of copending application Ser. No. 431,419, filed onNov. 3, 1989, now U.S. Pat. No. 4,964,461.

FIELD OF THE INVENTION

This invention relates to organically crosslinked polymeric gels and tothe use thereof in controlling the permeability of subterraneanoil-bearing formations and, more particularly, to a method forcontrolling the rate at which gelation takes place in order to controlthe permeability of the oil-bearing formations more effectively.

BACKGROUND OF THE INVENTION

Generally, in the production of oil from subterranean formations, only asmall fraction of the total formation oil can be recovered through theuse of primary recovery methods which utilize only the natural forcespresent in the reservoir. To recover additional oil, a variety ofsupplemental production techniques have been developed. In thesesupplemental techniques, commonly referred to as secondary or tertiaryrecovery operations, a fluid is introduced into the oil-bearingformation in order to displace oil to a production system comprising oneor more production wells. The displacing or "drive" fluid may be anaqueous liquid such as brine or fresh water, a gas such as carbondioxide, steam or dense-phase carbon dioxide, an oil-miscible liquidsuch as butane, or an oil and water-miscible liquid such as an alcohol.Often, the most cost-effective and desirable secondary recovery methodsinvolve the injection of steam. In practice, a number of injection andproduction wells will be used in a given field arranged in conventionalpatterns such as a line drive, a five spot or inverted five spot, or aseven spot or inverted seven spot.

In the use of the various flooding techniques, it has become a commonexpedient to add various polymeric thickening agents to the drive fluidto increase its viscosity to a point where it approaches that of the oilto be displaced, thus improving the displacement of oil from theformation. The polymers used for this purpose are often said to be usedfor "mobility" control.

Another problem encountered is that certain injected drive fluids may bemuch lighter than the reservoir fluids and thus separate by gravity,rising toward the top of the flowing region and resulting in thebypassing of the lower regions. This phenomena is known as gravityoverride.

Also encountered in the use of the various flooding techniques is asituation caused by the fact that different regions or strata often havedifferent permeabilities. When this situation is encountered, the drivefluid may preferentially enter regions of higher permeability due totheir lower resistance to flow rather than the regions of lowpermeability where significant volumes of oil often reside.

It therefore is often desirable to plug the regions of highpermeability, or "thief" zones, either partly or entirely, so as todivert the drive fluid into regions of lower permeability. Themechanical isolation of these thief zones has been tried but verticalcommunication among reservoir strata often renders this methodineffective. Physical plugging of the high permeability regions bycements and solid slurries has also been tried with varying degrees ofsuccess; however, these techniques have the drawback thatstill-productive sites may be permanently closed.

As a result of these earlier efforts, the desirability of designing aslug capable of sealing off the most permeable layers so that the drivefluid would be diverted to the underswept, "tighter" regions of thereservoir, became evident. This led to the use of oil/water emulsions,as well as gels and polymers for controlling the permeability of theformations. This process is frequently referred to as "floodconformance" or "profile control", a reference to the control of thevertical permeability profile of the reservoir. Profile control agentswhich have been proposed include oil/water emulsions, gels, e.g.,lignosulfate gels and polymeric gels, with polymeric gels being the mostextensively applied in recent years.

Among the polymers so far examined for improving flood conformance arepolyacrylamides, polysaccharides, celluloses, furfural-alcohol andacrylic/epoxy resins, silicates and polyisocyanurates. A major part ofthis work has been conducted with the polyacrylamides, both in theirnormal, non-crosslinked form, as well as in the form of metal complexes,as described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and4,413,680. In either form, the beneficial effects derived from thesepolyacrylamides seem to dissipate rapidly due to shear degradationduring injection and sensitivity to reservoir brines, low pH and hightemperature. To overcome these problems and to achieve deeper polymerpenetration into the reservoir, dilute solutions of these polymers havesometimes been injected first and then complexed in situ.

Another group of polymeric thickeners which has received considerableattention for use in improving flooding are polysaccharides,particularly those produced by the action of bacteria of the genusXanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and3,383,307 disclose a process for mobility control by the use ofpolysaccharides.

U.S. Pat. Nos. 3,741,307, 4,009,755 and 4,069,869 disclose the use ofpolysaccharides in the control of reservoir permeability. U.S. Pat. No.4,413,680 describes the use of crosslinked polysaccharides for selectivepermeability control in oil reservoirs.

U.S. Pat. No. 3,908,760 describes a polymer waterflooding process inwhich a gelled, water-soluble Xanthomonas polysaccharide is injectedinto a stratified reservoir to form a slug, band or front of gelextending vertically across both high permeability and low permeabilitystrata. This patent also suggests the use of complexed polysaccharidesto block natural or man-made fractures in formations.

Another type of polysaccharide which has been experimented with in thearea of profile control is the non-xanthan, heteropolysaccharide S-130.S-130 is a member of a group of welan gums and is produced byfermentation with a microorganism of the genus Alcaligenes. Anotherwelan gum heteropolysaccharide, known as S-194, is also produced byfermentation with a microorganism of the genus Alcaligenes. A notablecharacteristic of the heteropolysaccharide S-130 is that it develops ahigh viscosity in saline waters. This is particularly so in brines whichcontain divalent cations such as Ca²⁺ and Mg²⁺ or monovalent cationssuch as Na⁺ and K⁺. U.S. Pat. No. 4,658,898 discloses the use of welangum S-130 in saline waters. Crosslinking with trivalent cations, such aschromium, aluminum, zirconium and iron is also disclosed. Additionally,crosslinking with organic compounds containing at least two positivelycharged nitrogen atoms is disclosed in U.S. Pat. No. 4,658,898; whileSer. No. 283,399, filed on Dec. 12, 1988, discloses welan gumscrosslinked with phenolic resins or mixtures of phenols and aldehydes.

The use of various block copolymers for mobility control inwaterflooding operations is described in U.S. Pat. Nos. 4,110,232,4,120,801 and 4,222,881. Chung et al., U.S. Pat. No. 4,653,585, disclosethe use of block copolymers, which may be crosslinked with polyvalentmetal ions, for use as permeability control agents in enhanced oilrecovery applications.

While a number of the different compositions discussed have beenproposed for permeability control, some of these compositions may beunsuitable for use as permeability control agents under certaincircumstances. For example, the polymers of Chung et al, may not beeffectively crosslinked with polyvalent metal ions under all conditionsencountered in the enhanced oil recovery applications, e.g., in acidicconditions commonly found in carbon dioxide (CO₂) flooding operations.Polyacrylamides display instability in the presence of high brineconcentration at temperatures over 70° C. Xanthan gums are very brinetolerant but display thermal instability, even at temperatures below 60°C. Still, other polymers are unsuitable as permeability control agentswhen used in conjunction with steam flooding operations. This is due tothe fact that they lose their structural integrity at the hightemperatures generated during such operations. In view of the severeconditions which include both high brine concentrations, elevatedtemperatures or both, so-called hostile environment polymers, such asthose marketed by the Phillips Petroleum Company of Bartlesville, OK andthe Hoechst Celanese Corporation of Somerville, NJ have been developed.

One problem that has continually attended the use of polymeric mobilityand profile control agents is that thickened aqueous solutions, such asthe polysaccharide solutions, may be more difficult to inject into thereservoir than less viscous solutions. Also, the shear conditionsattendant during injection may degrade the polymer and reduce itseffectiveness upon entering the reservoir. To overcome injectivityproblems, U.S. Pat. No. 3,208,518 proposes the use of polymer solutionsof controlled pH which undergo a delayed increase in viscosity after thesolution enters the formation and the pH changes by neutralization ofacidic or basic constituents in the solution by materials present in thereservoir.

In general, there are two basic ways to deliver polymer gels into theformation. The first method is to inject gelled polymer into theformation. This is the so-called surface gelation method. The advantageof this method is that the polymer will enter the loose, more highlypermeable zone in preference to the tighter, low permeability zone, dueto the high viscosity of the gelled polymer. Another advantage is thatgelation is ensured since the gel is prepared at the surface. Thedisadvantage of this method is that the polymer gel will probably notpenetrate far enough to block a high pore volume of the designated zoneat low pumping pressures and low pumping rates. This is particularly sowhen the pressure drop occurs rapidly within a small radius of theinjection wellbore. At high pumping pressures and flow ratres, there areincreased risks of fracturing the reservoir and degrading the gelstructure by high shear forces, as those skilled in the art will readilyunderstand.

The second method is the so-called in situ gelation method, in whichseparate slugs of polymer, one containing an inactive crosslinker (suchas dichromate), the other, an activator (reducing agents such asthiourea and bisulfite), are injected sequentially into the reservoir.Gelation occurs when the two parts meet in the reservoir. With thismethod, shear degradation is reduced and the penetration of polymer isimproved because of the lower viscosity of the ungelled polymer.However, because of its low viscosity, the non-crosslinked polymer slugcan also enter the tight zone and cause its blockage, defeating thepurpose of the profile control treatment. Another disadvantage of thismethod is that there is no guarantee that the two slugs of treatmentwill be placed in the same area and mix well enough to form a stronggel.

To improve upon the aforementioned polymer delivery methods, a methodfor delivering gelled polymer into the formation in a manner whichensures the formation of a strong gel when the polymer is correctlyplaced in the formation and which avoids the problems associated withhigh injection pressures, pumping rates and shear forces would bedesirable. U.S. Pat. No. 4,606,407, the inventor of which is also theinventor of the present subject matter, discloses a method in whichpolymers are gelled in a controlled manner through the use of rapid anddelayed polyvalent metal gelling agents. The gelling agents disclosedare capable of forming two or more coordinate bonds with donor atoms inthe polymers. Polymers disclosed within U.S. Pat. No. 4,606,407 ashaving the requisite donor atoms for forming coordinate linkages includepolyacrylamides, other acrylic polymers and polysaccharides. In thepractice of the method of U.S. Pat. No. 4,606,407, a solution ordispersion of the polymer is first lightly gelled on the surface throughthe use of the rapid polyvalent metal crosslinking agent. The delayedpolyvalent metal crosslinking agent is also added to the solution ordispersion so as to effect complete gelation at a later period of timewhen the desired depth of penetration has been achieved. U.S. Pat. No.4,606,407 is hereby incorporated by reference in its entirety for allthat it discloses.

While transition metal-complexed polymer gels have been successful inmany profile control applications, several limitations may interferewith their use in the preparation of suitable gel-forming compositions.One limitation is that each metal is reactive only to certainfunctionalities. For example, Al, Cr, and Zr are reactive only to amideand carboxyl groups, while Ti is reactive to hydroxyl groups. A propermatch of the polymer with the appropriate metal crosslinker must beconsidered. There is presently no known general metal crosslinker forall types of polymeric materials. Carbonate, bicarbonate, and sulfateanions are known to interfere with the gelation of Cr, Zr and Al.Another limitation is that pH control is important for most metalcrosslinking reactions. It is easy to control pH when the gel isprepared at the surface but, as can be appreciated, such control can bevastly more difficult when an in situ gelation process is utilized.Furthermore ligand-metal bond formation and stability may be affected byhigh ionic strength and the temperature of the reservoir brine. Atsubstantially high brine concentrations and high temperatures, metalligand bonds can dissociate due to unfavorable equilibria.

Therefore, what is needed is a method for delivering gelled polymer intothe formation in a manner which ensures the formation of a strong gelwhen the polymer is correctly placed in the formation, irrespective ofreservoir type and specific conditions. The method should also avoid theproblems associated with high injection pressures, pumping rates andshear forces.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for usewith a polymer which is crosslinkable by reaction with an amino resin,which comprises the steps of determining a period of time within whichfull gelation of the polymer is to be achieved; preparing a gel-formingaqueous-based mixture comprising the polymer, a rapid amino resincrosslinking agent and a delayed amino resin crosslinking agent, thegel-forming aqueous-based mixture capable of complete gelation withinthe period of time so determined; partially gelling the aqueous-basedmixture by reacting the polymer with the rapid amino resin crosslinkingagent which is effective to complete the partial gelation bycrosslinking the polymer within four hours; and fully gelling theaqueous-based mixture by reacting the polymer with the delayed aminoresin crosslinking agent which is effective to complete the fullgelation by crosslinking the polymer within the period of time sodetermined. When used in a process to enhance the recovery of oil, thepolymeric mixture, which may be in the form of a solution or dispersion,is initially gelled to a limited degree on the surface by the rapidamino resin crosslinking agent. The delayed amino resin crosslinkingagent will effect complete gelation at a later time when the desireddepth of penetration is achieved within the subterranean formation. Thetotal concentration of amino resin crosslinking agents will determinethe final strength of the fully gelled polymer.

The initial partial gelation increases the viscosity of the polymersolution or dispersion to impart selectivity to enter only the highpermeability zones of the subterranean oil-bearing formation. At thesame time, since it is only partially gelled, the polymer mixture isable to be pumped deep into the formation, with greater ease, than afully gelled mixture. Full gelation by the delayed amino resincrosslinking agent will occur when the planned treatment depth isachieved. Since the full gelation is not developed during the deliveryperiod, unnecessary degradation of gel structure by shear forces isminimized. Such a process, wherein gelation is tailored to a specificapplication to achieve the aforementioned desirable properties, is saidto be a programmed gelation process. A method of enhancing the recoveryof oil from a subterranean oil-bearing formation is also provided.

DETAILED DESCRIPTION OF THE INVENTION

In the method of the present invention, a viscous or thickened liquidcomprising a partly gelled polymer is injected into a subterraneanoil-bearing formation in order to block the more highly permeableregions in a selective manner. The liquid which is injected is asolution or dispersion of the partly gelled polymer in water. For thepurposes of this description, the liquid will subsequently be referredto as a solution despite the fact that in some cases the polymer willactually be present as a dispersion, rather than a solution in the truesense of the term.

The polymer is injected into the formation through an injection wellwhich extends from the surface of the earth into the formation. Inaddition, a production well is situated on a horizontal distance oroffset from the injection well so that, once the polymer has been placedin the formation to control the permeability and the flooding operationbegun in the normal manner by injecting the flooding fluid, e.g. waterthrough the injection well, recovery of the oil displaced by theflooding fluid can be made through the production well.

Any water-soluble or water-dispersible polymer capable of formingaqueous gels in the presence of an organic crosslinking agent isenvisioned for use in the practice of the present invention. The polymerwhich is used to produce the desired gel may be of natural or syntheticorigin. Because the multiple step gelation reaction depends upon theformation of coordinate crosslinkages, the polymer should containfunctional groups such as --NH₂, --CONH₂, --OH, --SH, or --COOH. Suchfunctional groups may be introduced into the polymer either by the useof appropriately substituted monomers, by grafting techniques or byreaction of a pre-formed polymer with a suitable reagent for introducingthe desired functional groups. As can be appreciated by those skilled inthe art, the aforementioned reactive groups are not meant as alimitation as to the types of polymers useful in the practice of thepresent invention, but are presented for purposes of example.

Suitable polymers include acrylic polymers, e.g. polyacrylic acid,polyacrylic acid esters, polyacrylamide, polymethacrylic acid,polymethacrylic acid esters, copolymers of unsaturated carboxylic acids,such as acrylic acid or methacrylic acid with olefins such as ethylene,propylene, and butylene, vinyl polymers such as polyvinyl acetate andpolyvinyl alcohol, polymers of unsaturated dibasic acids and anhydridessuch as maleic anhydride, and their copolymers with other monomers suchas ethylene, propylene, styrene and methylstyrene. Other exemplarypolymers are described in U.S. Pat. No. 3,208,518, which is herebyincorporated by reference for such details.

Preferred polymers include the various polyacrylamides and relatedpolymers which are either water-soluble or water-dispersible and whichcan be used in an aqueous medium with the gelling agents describedherein to yield an aqueous gel. These can include the varioussubstantially linear homopolymers and copolymers of acrylamide andmethacrylamide. By substantially linear is meant that the polymers aresubstantially free crosslinking between the polymer chains. The polymerscan have up to about 50 percent of the carboxamide groups hydrolyzed tocarboxyl groups. However, as the degree of hydrolysis increases, thepolymers often become more difficult to disperse in brines, especiallyhard brines. As used herein, unless otherwise specified, the term"hydrolyzed" includes modified polymers wherein the carboxyl groups arein the acid form and also such polymers wherein the carboxyl groups arein the salt form, provided such salts are water-dispersible. Such saltsinclude the ammonium salts, the alkali metal salts, and others which arewater-dispersible. Hydrolysis can be carried out in any suitablefashion, for example, by heating an aqueous solution of the polymer witha suitable amount of sodium hydroxide.

Examples of copolymers which can be used in the practice of theinvention include the water-dispersible copolymers resulting from thepolymerization of acrylamide or methacrylamide with an ethylenicallyunsaturated monomer. It is desirable that sufficient acrylamide ormethacrylamide be present in the monomer mixture to impart to theresulting copolymer the above-described water-dispersible properties.Any suitable ratio of monomers meeting this condition can be used. Underproper conditions of use, examples of suitable ethylenically unsaturatedmonomers include acrylic acid, methacrylic acid, vinylsulfonic acid,vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, vinyl acetate,acrylonitrile, methyl acrylonitrile, vinyl alkyl ether, vinyl chloride,maleic anhydride, vinyl-substituted cationic quaternary ammoniumcompounds, and the like. Various methods are known in the art forpreparing said copolymers. For example, see U.S. Pat. Nos. 2,625,529,2,740,522, 2,727,557, 2,831,841, and 2,909,508. These copolymers can beused in the hydrolyzed form, as discussed above for the homopolymers.

A group of copolymers useful in the practice of the present inventionare the copolymers of acrylamide or methacrylamide and a monomer such asthe well known 2-acrylamido-2-methyl-propanesulfonic acid (AMPS®)monomer. (AMPS® is the registered trademark of the Lubrizol Corporationof Cleveland, OH.) Useful monomers, such as the AMPS® monomer, andmethods for their preparation are described in U.S. Pat. Nos. 3,507,707and 3,768,565, the disclosures of which are incorporated by reference.The AMPS® monomer is commercially available from the LubrizolCorporation. The alkali metal salts, such as sodium2-acrylamido-2-methylpropane sulfonate are also useful in the practiceof this invention. These are also readily available.

Copolymers of acrylamide with said AMPS® monomer, and/or its sodiumsalt, are known and useful in the practice of this invention. For anexample of such a copolymer, see the above-mentioned U.S. Pat. No.3,768,565. A number of these copolymers are available from HerculesIncorporated, Wilmington, Del.; for example, Hercules SPX-5024, a 90:10acrylamide/AMPS® sodium salt copolymer; Hercules SPX-5022, an 80:20acrylamide/AMPS® sodium salt copolymer; Hercules SPX-5023, a 50:50acrylamide/AMPS® sodium salt copolymer; and Hercules SPX-5025, a 30:70acrylamide/AMPS® sodium salt copolymer.

Another group of copolymers useful in the practice of the invention arethe copolymers of acrylamide or methacrylamide with a monomer such asthose which are the subject of U.S. Pat. No. 3,573,263, the disclosureof which is incorporated by reference in its entirety. These usefulmonomers include the well known commercially available material(acryloyloxyethyl) diethylmethyl ammonium methyl sulfate, commonlyreferred to as DEMMS and the commercially available material(methacryloyloxyethyl) trimethylammonium methylsulfate also known asMIMMS.

Copolymers of acrylamide with said DEMMS monomer are commerciallyavailable, for example, an 80:20 acrylamide/DEMMS copolymer. Copolymersof acrylamide with said MtMMS monomer are also commercially available,for example, Hercules Reten® 210, a 90:10 acrylamide/MTMMS copolymer;and Hercules Reten® 220, an 80:20 acrylamide/MTMMS copolymer.

A particularly preferred polymeric material for use in the practice ofthis invention is the class of high molecular weight vinyl lactumpolymers and copolymers disclosed in U.S. Pat. No. 4,644,020, which ishereby incorporated herein in its entirety. An example of a commerciallyavailable copolymer of this type is Phillips HE-B®, which is a copolymerof N-vinyl-2-pyrrolidone and acrylamide. This thermally stable, brinetolerant copolymer is available from Phillips Petroleum Company, Inc.,of Bartlesville, OK.

A preferred class of biopolymers which may be used include thepolysaccharides produced by the action of bacteria of the genusXanthomonas on a carbohydrate. The Xanthomonas polysaccharides, theirmethod of preparation, their use in various applications in thepetroleum industry are well known and are described, for example, inU.S. Pat. Nos. 3,243,000, 3,305,016, 3,208,518, 3,810,882 and 4,413,680,to which reference is made for disclosures of these materials, theirpreparation and their use. Other polymers of natural origin that may beused include cellulose polymers, e.g., the hydroxyalkyl celluloses andcarboxyalkyl celluloses and their alkali metal and ammonium salts, asdescribed in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680, to whichreference is made for a detailed description of these polymers.

A particular polysaccharide which is commercially available and ispreferred for use in the present invention is the ionic polysaccharideB-1459 produced by fermentation of glucose with the bacteriumXanthomonas Campestris (NRRL B-1459, U.S. Department of Agriculture).This polysaccharide is produced by culturing the bacterium XanthomonasCampestris in a well aerated medium having a pH of about 7 whichcontains commercial glucose, organic nitrogen sources, dipotassiumhydrogen phosphate and appropriate trace elements. This polymer isavailable from the Kelco Chemical Company under the trade name "Kelzan",from Pfizer under the trade name "Flocon" and from other commercialsources.

Another biopolymer which may be employed in the practice of theinvention disclosed herein is the non-xanthan welan gumheteropolysaccharide biopolymer S-130 produced by fermentation underaerobic conditions of a bacterium of the Alcaligenes species, ATCC31555. This polysaccharide is described in U.S. Pat. No. 4,342,866 towhich reference is made for a description of it and of the method bywhich it may be produced. S-130 is commercially available from the KelcoOil Field Group, a division of Merck and Co., Inc.

The polymers are generally used at concentrations ranging from 1,000 to5,000 ppm in order to achieve the desired gel consistency; in mostcases, however, concentrations of 1,000 to 3,000 ppm will be adequateand about 2,000 ppm is normally preferred, although reservoir conditionsmay require other concentrations.

The polymer is initially dissolved or suspended in water and is thegelled in two stages, preferably by the use first of a rapidcrosslinking agent and second, by the use of a delayed crosslinkingagent. The rapid crosslinking agents can generally be considered asthose which would, if present at a sufficient concentration, gel thepolymer completely within four hours. Delayed crosslinking agents, bycontrast, will be those which would gel the polymer completely over aperiod of time in excess of four hours and normally require a period ofa few days or longer for complete gelation. The period of time which thecrosslinking agent requires to effect gelation is not, however,critical, because the objective underlying the use of two differentagents is to permit the introduction of a partly gelled polymer into theformation and, subsequently, to complete gelation after the polymer isin place in the formation so that a firm, crosslinked gel is formed. Ascan be appreciated, it would therefore be possible to employ a delayedcrosslinking agent to cause the initial gelation but, because this wouldcause an inordinate delay in the progress of the work, it will normallynot be employed. Generally, the rapid crosslinking agents with agelation time of not more than four hours will cause sufficient gellingto impart selectivity to the overall solution to be injected within aconveniently short period of time. Because the effect of the rapidcrosslinking agent is complete within a reasonably short period of time,it is possible to wait until this partial crosslinking is complete, toensure that the solution which is injected has the optimum properties,e.g. selectivity, viscosity, shear stability, for injection. If acrosslinking agent with a longer gelation time were initially used, thegelation would continue over an extended period of time so that if anyundue delays in the injection of the slug were encountered, the portionsof the solution which would be injected later would be more highlygelled than the initial portions and so the injection process might notbe performed under optimal conditions.

The particular crosslinking system employed in the present invention isone selected from the family of amino resins. Amino resins are preparedby reacting formaldehyde with urea or melamine. These resins, also knownto those skilled in the art as aminoplasts, are very effectivecrosslinking agents. When used with the preferred polymers describedabove, stable gels may be obtained, even at pH conditions under whichpolyvalent metal crosslinking systems may be ineffective or formunstable gels. Thus, the crosslinking agents disclosed herein form gelswhich are stable even at acidic formation conditions, e.g., at pH valuesof about 5.5 or less, such as the conditions encountered in CO₂ floodingoperations.

The amino resin, to be effective in the practice of the presentinvention, must be soluble or dispersible in an aqueous medium.Non-limiting examples of resins which can be used are melamineformaldehyde, urea formaldehyde, ethylene and propylene ureaformaldehyde, triazone, uran and glyoxyl resins. The amount of aminoresin required for polymer crosslinking is about 0.1:1 to about 10:1 byweight of the polymer to the amino resin. The manner of preparation isdescribed in Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, ThirdEdition, Volume 2, John Wiley and Sons, 1978, at pages 440-467, thecontents of which are incorporated herein by reference in theirentirety.

The crosslinking system preferred for use in the programmed gelationmethod of the present invention is one selected from the family ofmelamine resins. Melamine resins are derived from a reaction of melamineand formaldehyde at a molar ratio of melamine to formaldehyde of betweenabout 1 to about 6, with a ratio of between about 3 to about 6 commonlyemployed. Such resins form the group of rapid crosslinking agents foruse in the practice of the present invention and have the followingstructure: ##STR1## wherein a, b, c=0, 1, 2; and, 0<a+b+c<6. Suchmelamine formaldehyde resins can crosslink a polymer from the group ofpolymers disclosed herein in a short period of time, normally four hoursor less. The methylol group, --CH₂ OH, is known to be reactive tovarious functionalities, such as --NH₂, --CONH₂, --OH, --SH , or --COOHgroups.

To modify the reactivity and solubility in organic solvents of thepreferred melamine resins, the methylol groups can be alkylated withmethyl, ethyl, propyl or butyl groups. The alkylated melamine resinsform the group of useful delayed crosslinking agents required for thepractice of the present invention. Such resins have the property thatthey will crosslink the group of disclosed polymers over a period oftime in excess of four hours to form stable aqueous gels. A methylatedmelamine formaldehyde resin has the following structure: ##STR2##wherein a, b, c=0, 1, 2; and, 0<a+b+c<6. The preparation of alkylatedmelamine formaldehyde resin is well known and documented in the variouspreparative polymer manuals, such as Polymer Synthesis, by Sandler andKaro, published by Academic Press (1977), incorporated by reference forthose details.

A melamine formaldehyde resin such as a trimethylol melamine can reactwith the aforementioned functional groups at room temperature at a pH ofabout 8 or less. A methylated melamine formaldehyde resin such as ahexamethyl-hexamethylol melamine requires higher temperatures and anacid catalyst to become an active crosslinker. A partially methylatedmelamine formaldehyde resin will be partially reactive at lowtemperatures without an acid catalyst but will become fully reactive inthe presence of such a catalyst. A latent, heat activated catalyst ispreferred for use in the practice of the method of the presentinvention. Particularly preferred catalysts include anhydrides ofcarboxylic acids and the ammonium salts of strong acids, such asp-toluenesulfonic acid and dihydrophosphate.

Melamine formaldehyde resins used in the practice of the presentinvention can be commercial products such as the partially methylatedand hexamethoxymethyl resins produced by the American Cyanamid Companyof Wayne, NJ and sold under the trademarks "Cymel®" and "Parez®". Asmentioned previously, the resin employed must be one that is soluble ordispersible in an aqueous medium. For the alkylated melamineformaldehyde resins, it is known that methyl- and ethyl-group alkylationdoes not adversely affect solubility, while propyl- and butyl-groupalkylation will begin to detract from the resin's ability to bedispersible in an aqueous medium.

As described above, the purpose of the initial, partial gelation is togive the polymer sufficient selectivity to prevent it from entering themore permeable regions of the formation but, at the same time to keepthe gel strength low enough so that high injection rates and pressuresare unnecessary and to permit the polymer to be injected deeply into theformation so that a large volume of the more highly permeable regionsare plugged. The degree of gelation at this stage should therefore becontrolled so as to meet these objectives. Because this will depend uponthe permeabilities which are encountered in the formation and to theextent to which the more permeable regions are to be selectivelyplugged, the extent of gelation will be selected by empirical means andcontrolled by the amount of crosslinking agent used. Because a furtherdegree of gelation is to take place once the polymer is in place in theformation, the amount of crosslinking agent used at this stage (relativeto the polymer) should not be so great as to be capable of taking up allof the available crosslinking sites on the polymer. The total amount ofthe polymer to be employed will, of course, depend upon the volume ofthe formation which is to be treated and will itself be determined byempirical means. The gel strength and size of the slug to be selectedfor a particular field application will depend upon reservoir and fluidproperties, the degree of stratification, the extent of multi-zonalinjection and commingled production, variation and symmetry of wellspacing, and oil/water mobility ratio. Because the final strength of thepolymer gel is determined by the total amount of crosslinking, the finalgel strength will be dependent upon the nature of the polymer, thenumber of crosslinking sites available, and the total amount ofcrosslinking agents used, up to the necessary amount to bring aboutcomplete crosslinking. Thus, in general the final gel strength of agiven polymer may be determined by the total amount of crosslinkingagent used, relative to the total amount of the polymer. Higher gelstrengths permit higher flooding pressures to be employed without riskof polymer disintegration.

As a result of the number of empirical factors, it is not possible toindicate the exact amounts of polymer and crosslinking agents which willbe used in all applications and at all times. However, as a generalguide, when using the preferred Xanthomonas polysaccharides, the amountof the polymeric material will range from about 1000 to about 4000 ppmin the solution and, preferably between about 1500 to about 3000 ppm.For the preferred melamine formaldehyde crosslinking agents, the amountrequired for complete crosslinking will generally be from about 0.1 to 1to about 10 to 1 by weight of the polymer to the total amount (rapidplus delayed crosslinking agents) of melamine resins, with the totalamount of rapid and delayed crosslinking agents adjusted to providecomplete polymer crosslinking. In general, the amount of viscous liquidwhich may be injected into the stratified formation may be from about10% to about 100% of the pore volume of the more highly permeable strataor stratum.

When a xanthan or welan gum polysaccharide is the polymer selected foruse, the partly crosslinked solution which is injected into thestratified formation is capable of undergoing a reversibleshear-thinning effect and this property may be exploited in theplacement of the slug, aided by the pressure gradient around theinjection well. In the vicinity of an injector, the flow rate and theassociated pressure gradient are at a maximum and drop off rapidly asthe radical distance from the wellbore increases. Thus, as the injectedpolymer solution flows outward, its apparent viscosity will be initiallylow and hence the slug can be readily injected. At a locationsufficiently far away from the injector, for example, about 30 feet, theflow rate and pressure gradient are much reduced and the viscosityreturns to its low-shear, higher value. This increased viscosityarrests, and in some cases, stops altogether the movement through theformation of the polymer slug. The delayed gelling effect then takesplace to form a final polymer gel of high strength. The combination ofthe shear thinning effects and the programmed crosslinking of thepolymer serves to provide a selective placement of viscous slugs withinthief zones.

The injected fluid will be proportioned into the various reservoirstrata according to their effective permeabilities and flow capacities.The blocking of the most permeable flow channels will lead to thediversion of the flooding fluid to the underswept portions the reservoirand, in turn, to improved oil recovery.

The present invention is further illustrated by the followingnon-limiting prophetic example:

EXAMPLE

To treat a well of moderate size, it is determined that 1,000 barrels ofprofile control treatment are necessary. Flocon® 4800 xanthanpolysaccharide, obtained from the Pfizer Corporation of Easton, PA, isselected as the polymer for use in the treatment. A two day period isestimated as being required to place the treatment within the desiredlocation based upon reservoir conditions. A melamine formaldehydecrosslinking system is designed to achieve the desired programmedgelation of the polymeric material from the following ranges ofmaterials:

                  TABLE                                                           ______________________________________                                        Mixture of Crosslinking Agents for Programmed Gelation                        Component    Proportion Function                                              ______________________________________                                        Methylol Melamine                                                                           5-50%     Partial gelation to impart                            Formaldehyde            viscosity and selectivity                             Methylated Melamine                                                                        50-95%     Gelation occurs in the for-                           Formaldehyde            mation to give the gel full                                                   strength and rigidity                                 Acetic Anhydride                                                                           0.01-1%    Heat activated catalyst                               ______________________________________                                    

The melamine formaldehyde crosslinking system is utilized at aconcentration effective to fully crosslink the polymer. The initial gelstrength is determined by the relative amount of methylol melamineformaldehyde (rapid crosslinking agent), the amount being within thepreferred range cited in the table above. As can be appreciated, higherinitial gel strength is obtained with higher concentrations of methylolmelamine formaldehyde. Total melamine formaldehyde resin concentration(methylol melamine formaldehyde plus methylated melamine formaldehyde)determines final gel strength. Gelation rate is controlled by theconcentration of acid catalyst used. As discussed above, to tailor thegelation reaction still further, a partially methylated melamineformaldehyde resin can be substituted in whole or in part for the fullymethylated melamine formaldehyde resin listed in the table.

The gel-forming solution is prepared on the surface in a suitable tankequipped with suitable mixing means and then pumped down the well andinto the formation employing conventional equipment for pumping suchcompositions. The solution is placed in the formation at a rate of about20 barrels per hour and requires about two days for placement. A rigidstable gel is formed at the end of the placement period.

It is within the scope of the invention to prepare these compositionsjust prior to being pumped down the well, so long as the rapidcrosslinking agent has sufficient time to partially gel the polymer. Forexample, a solution of the polymer in water can be prepared in a tankadjacent to the wellhead. Pumping of this solution through a conduit tothe wellhead can then be started. Then, downstream from the tank, asuitable connection can be provided for introducing the crosslinkingsystem. As will be understood by those skilled in the art, the rate ofintroduction of the crosslinking agents into the conduit will dependupon the pumping rate of the polymeric solution through the conduit. Anyof the above-mentioned orders of addition can be employed in such atechnique. Mixing orifices or baffles can be provided in the conduit, ifdesired.

Where it is desired to obtain increased sweep efficiency, the process ofthis invention can be used to plug a previously swept portion of aformation. The process may additionally be carried out periodically,when necessary, to achieve the desired permeability profile.

One application where the process of this invention can be utilized isduring a waterflooding process for the recovery of oil from asubterranean formation. After plugging the more permeable zones of areservoir using the process of this invention, a waterflooding processcan be commenced or resumed. U.S. Pat. No. 4,479,894, issued to Chen etal., describes one such waterflooding process. This patent is herebyincorporated by reference in its entirety.

Steamflood processes which can be utilized in conjunction with theprocess described herein are detailed in U.S. Pat. Nos. 4,489,783 and3,918,521 issued to Shu and Snavely, respectively. These patents arehereby incorporated by reference herein.

The process described herein can also be used in conjunction with acarbon dioxide flooding process, either alone, or in conjunction with acyclical steam stimulation in a heavy oil recovery process to obtaingreater sweep efficiency. Cyclic carbon dioxide steam stimulation can becommenced or resumed after plugging the more permeable zones of thereservoir using the process of this invention. A suitable process isdescribed in U.S. Pat. No. 4,565,249 which issued to Pebdani et al. Thispatent is hereby incorporated by reference in its entirety. Increasedsweep efficiency can be obtained when the process of this invention isused in combination with a carbon dioxide process for recovering oil.Prior to commencement or resumption of the carbon dioxide process, themore permeable zones are plugged in the manner disclose herein

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be utilized without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A process for the programmed gelation of apolymer which is crosslinkable by reaction with an amino resin, whichcomprises the following steps:(a) determining a period of time withinwhich full gelation of the polymer is to be achieved; (b) preparing agel-forming aqueous-based mixture comprising the polymer, a rapid aminoresin crosslinking agent and a delayed amino resin crosslinking agent,said gel-forming aqueous-based mixture capable of complete gelationwithin the period of time determined in step (a); (c) partially gellingthe aqueous-based mixture by reacting the polymer with the rapid aminoresin crosslinking agent which is effective to complete the partialgelation by crosslinking the polymer within four hours; and (d) fullygelling the aqueous-based mixture by reacting the polymer with thedelayed amino resin crosslinking agent which is effective to completethe full gelation by crosslinking the polymer within the period of timedetermined in step (a).
 2. The process of claim 1, wherein the polymeris selected from the group consisting of polyacrylamides,polysaccharides, heteropolysaccharides, cellulose ethers and mixturesthereof.
 3. The process of claim 2, wherein the rapid amino resincrosslinking agent is selected from the group consisting of melamineformaldehyde, urea formaldehyde, ethylene urea formaldehyde, propyleneurea formaldehyde, triazone, uron, glyoxyl and mixtures thereof.
 4. Theprocess of claim 3, wherein the delayed amino resin crosslinking agentis selected from the group consisting of melamine formaldehyde, ureaformaldehyde, ethylene urea formaldehyde propylene urea formaldehyde,triazone, uron, glyoxyl and mixtures thereof.
 5. The process of claim 4,wherein the melamine formaldehyde resin is derived from a reaction ofmelamine and formaldehyde at a molar ratio of melamine to formaldehydeof between about 1 to about
 6. 6. The process of claim 5, wherein therapid crosslinking agent is comprised of a methylol melamineformaldehyde resin.
 7. The process of claim 6, wherein the delayedcrosslinking agent is comprised of an alkylated melamine formaldehyderesin.
 8. The process of claim 7, wherein the alkylated melamineformaldehyde resin is alkylated with methyl, ethyl, propyl or butylgroups.
 9. The process of claim 8, wherein the alkylated melamineformaldehyde resin is methylated or partially methylated melamineformaldehyde resin.
 10. The process of claim 8, wherein the gel-formingaqueous-based mixture further comprises an acid catalyst.